Toxic volatile organic compounds (VOCs) such as formaldehyde are increasingly prevalent in indoor air in both residential and in industrial settings. Formaldehyde is used in manufacturing of resins, plastics, rubber, textile finishes and especially wood based products. Ureaformaldehyde and melamine-formaldehyde resins are known for the release of formaldehyde. In particular, formaldehyde is released by the offgassing from urea-formaldehyde foam that is used as insulation for homes and businesses as well as carpet paddings. Formaldehyde is also released by the offgassing from urea-formaldehyde bonded chipboard that is used in furniture manufacturing and for building materials. Other undesirable toxins emitted from wood are monoterpenes which are a natural byproduct of wood devolatilization.
Major terpenic VOCs which are the natural toxic byproducts of wood devolatilization when wood is exposed or degraded include .alpha.-pinene, .beta.-pinene, limonene, camphene, and fenchone.
The Clean Air Act of 1990 requires minimization of discharges to the environment and limiting VOC emissions. The detoxification of hazardous compounds generated within a given medium will require an oxidative process to "burn off" the pollutant species.
A substance that can mediate oxidation of the environmental pollutants to deep oxidation by-products such as CO.sub.2 and H.sub.2 O would be desirable. In general, this implies the use of oxidizing agents such as ozone and hydrogen peroxide. However, these two oxidizing agents alone are not capable of total destruction of the contaminants.
Currently, the most advanced technology for the reduction of VOCs is regenerative catalytic oxidation(RCO).
Coupling light with an oxidant such as ozone O.sub.3, hydrogen peroxide H.sub.2 O.sub.2, titania TiO.sub.2, and others referred to as advanced oxidation processes (AOPs) has been shown to accomplish deep oxidation of all carbonaceous species to CO.sub.2. In heterogeneous photocatalysis, the oxidation process is aided by using photocatalysts such as TiO.sub.2, zinc oxide (ZnO) and the like. For example, TiO.sub.2 particles are readily activated upon exposure to near UV radiation (wavelengths below 365 nm) producing electron/hole (e.sup.- /h.sup.+) pairs on the semiconductor surface. The electrons and holes act as strong reducing and oxidizing agents that facilitate mineralization of the target organics via formation of active species such as superoxide ion radical (O.sub.2.sup.-.circle-solid.), hydroxyl radical (OH.sup..circle-solid.), and peroxyl radical (HO.sub.2.sup..circle-solid.) on the semiconductor surface. For example, conduction band electrons (e.sup.-.sub.CB) can reduce molecular oxygen to reactive radicals as follows:
e.sup.-.sub.CB +O.sub.2 .fwdarw.O.sub.2.sup.-.circle-solid. PA1 O.sub.2.sup.-.circle-solid. +H.sup.+ .fwdarw.HO.sub.2.sup.-.circle-solid. PA1 2 HO.sub.2.sup.-.circle-solid. .fwdarw.O.sub.2 +H.sub.2 O.sub.2 PA1 H.sub.2 O.sub.2 .fwdarw.2 OH.sup..circle-solid. PA1 h.sup.+.sub.VB +H.sub.2 O.fwdarw.H.sup.+
Table 1 depicts the relative oxidizing power of surface borne radicals compared with other commonly used oxidants. See Harris, J. C., Ozonation, In Unit Operations for Treatment of Hazardous Industrial Wastes, Noyes Data Corporation, Park Ridge, N.J., 1978. It can be seen that the oxidizing power of hydroxyl radicals is highest among all transhalogen oxidants and surpassed only by flourine. The valence band holes (h.sup.+.sub.VB) can also oxidize water to produce hydroxyl radicals, where the net effect can be complete oxidation or full mineralization of target organics.
TABLE 1 ______________________________________ Relative oxidation power of oxidizing species Oxidation potential Relative oxidation power Species (volts) (based on Cl .apprxeq. 1) ______________________________________ F 3.06 2.25 OH.sup..cndot. 2.80 2.05 atomic oxygen 2.42 1.78 O.sub.3 2.07 1.52 H.sub.2 O.sub.2 1.77 1.30 HO.sub.2.sup..cndot. 1.70 1.25 permanganate 1.70 1.25 hypochlorous acid 1.49 1.10 Cl 1.36 1.00 ______________________________________
The net effect is generally complete oxidation and full mineralization of target organics. Titania-catalyzed photooxidation processes combine light-assisted interactions of both homogeneous and heterogeneous nature. In addition, the combination of photons and the semiconductor photocatalyst results in new reaction pathways (i.e. photoreduction of oxygen by conduction band electrons and photooxidation by valence band holes) unavailable in non-catalytic photolysis. Reactive species (e.g. superoxide anion and hydroxyl radicals, etc.) are produced photolytically on the surface of the catalyst in the presence of water vapor and/or oxygen molecules eliminating the need for addition of active oxidants such as ozone and hydrogen peroxide. Also, the catalytically active surfaces that exist on titania influence the photodegradation kinetics and the extent of conversion of the target organic molecules. These factors combine to produce many opportunities and process flexibility to affect conversion efficiencies as well as the selectivities of the photocatalytic process. Unlike thermocatalytic processes, the photocatalytic process actually benefits from high moisture content of the media within which toxins reside as the hole oxidation of water generates highly reactive OH.sup..circle-solid. species that attack target organics.
Thus, the need exists to develop a more effective process for the detoxification of manufactured and wood borne toxic emissions such as formaldehyde and terpenes.